In a sport shoe, shock absorption upon foot strike is a vital consideration in protecting the lower extremity from injury. Distance runners usually impact the ground at the heel and with a force as high as three times the body weight. Most running shoes have been made with resilient elastomer (EVA) soles to at least partially cushion such impact. Other shoes have been constructed with shock absorption systems that include open or closed gas or liquid filled bladders. As these various soles are made thicker or contain more or larger air cells, bladders or tubes to improve shock absorption characteristics, the shoes become increasingly unstable. This instability is not particularly critical in a longitudinal direction. However, instability in a lateral direction adversely effects the function of the foot and leg in distance running and other sports and significantly increases the probability of acute and chronic lower extremity injury.
In distance running, from toe off until foot strike, the lower extremity is internally rotated, placed toward the midline of the body, and the foot is in a raised arch or supinated position. Most runners contact the ground with the outside edge of their shoes and for approximately ten to twenty percent of the total time the foot is on the ground, the foot continues to internally rotate. The foot longitudinal arch also lowers or pronates. This pronation, which occurs in the subtalar joint, allows the foot to act as a shock absorber and to become a mobile adapter to varying types of surfaces. The ground contact usually occurs with between two to four degrees of supination. The heel then angulates inwardly as the foot flattens about six degrees to a pronated position.
Many runners have a tendency to overpronate which makes subsequent raising of the longitudinal arch (supination) and external rotation of the foot to form a rigid lever for effective toe off more difficult. This "rolling" movement of the rear foot toward the inside (pronation) during midstance is exaggerated if the sole is particularly thick and soft or if the heel counter is unstable. This overpronation can lead to imbalance and overuse injuries which are common disabilities among runners. The subtalar joint functions as a mitered hinge with a diagonal or oblique axis. Thus, when the foot pronates excessively, the leg is forced to rotate inwardly to an excessive degree causing abnormal stress on the cartilage and musculotendinous components of the foot, leg and knee.
Repeated and continuous stress can cause numerous disorders. The additional load on the plantar fascia of the foot because of excessive pronation can result in plantar fascutes. "Shin splints" caused by traction on the posterior tibialis muscle and tendon which raises the longitudinal arch or tarsal tunnel syndrome caused by excessive friction on the posterior tibialis tendon under the medial malleolus, are pronation related disabilities. Torque on the achilles tendon initiates and aggravates achilles tendonitis. The abnormal internal rotation of the leg causes a stress on the structures of the medial aspect of the knee with pes anserinus bursitis being a common ailment. Misalignment of the quadraceps tendon can cause patellar compression syndrome due to the increased and abnormal pressure on the patellar cartilage.
Since foot impact is approximately two times body weight on level terrain and three times body weight in downhill running, a shoe sole that will deform to absorb the considerable energy of impact with minimal lateral instability is necessary to prevent injury. Several sport shoe manufacturers have attempted to solve this problem of lateral instability by various means including use of a stabilizing bar that traverses the midsole and heel counter (Converse), a stabilizing pillar in the midsole (Asics), a semirigid external heel counter that traverses the midsole (New Balance), and a stabilizing varus wedge in the midsole (Brooks). In all of these attempts to control lateral instability, the shock absorption qualities of the midsole are significantly compromised. This is because the stabilizer decreases the potential vertical compression or deformation of the midsole available for energy absorption. Also, these rigid or semirigid stabilizers cause the sole to "bottom out". Although most distance runners contact the ground with the outside edge of their shoes, this contact force is small. By the time the force reaches twice the body weight the distribution is centered approximately twenty five percent forward of the shoe length from the heel and near the midline of the shoe. It is, therefore, essential that the shoe sole provide maximum vertical shock absorption toward the center of the heel while minimizing lateral instability.
Disclosures relating to shock absorption systems for shoes include the following U.S. Pat. Nos.:
______________________________________ 663,270 3,716,930 2,474,815 3,754,339 3,029,530 3,785,069 3,120,712 3,791,051 3,180,039 4,183,156 3,335,505 4,215,492 3,475,836 4,219,945 3,589,037 4,224,746 4,237,625 ______________________________________